Transition metal chalcogenide van der waals films, methods of making same, and apparatuses and devices comprising same

ABSTRACT

Provided are van der Waals (VDW) films comprising one or more transition metal chalcogenide (TMD) films. Also provided are methods of making VDW films. The methods are based on transfer of monolayer TMD films under vacuum, for example, using a handle layer. Also provided are apparatuses and devices comprising one or more VDW film.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.62/360,053, filed on Jul. 8, 2016, the disclosure of which is herebyincorporated by reference.

STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under NSF DMR-1120296awarded by the National Science Foundation and under FA2386-13-1-4118awarded by the Air Force Office of Scientific Research. The governmenthas certain rights in the invention.

FIELD OF THE DISCLOSURE

The disclosure generally relates to transition metal chalcogenide vander Waals films. More particularly the disclosure generally relates tomethods of making van der Waals films by stacking monolayer transitionmetal chalcogenide films.

BACKGROUND OF THE DISCLOSURE

Thin film processing with composition and thickness control is essentialfor modern semiconductor technology. Accordingly, reaching a fundamentallimit of controllability down to atomic level in large scale would allowus to design innovative artificial materials for practical applicationssuch as quantum electronics and photonics.

Currently, the uniform monolayer building block of TMDs in large scaleis available using metal-organic chemical vapor deposition (MOCVD).However, existing methods to assemble the VDW films show poorcontrollability and/or scalability up to date. For example, directgrowth of multilayer VDW films uniform in large scale is not preferredbecause there is only weak driving force to induce homogeneousnucleation on each layer. In parallel, layer-by-layer stacking usingexfoliated flakes is limited to micron-meter size without scalability.Furthermore, the cleanliness at the stacking interface is not guaranteedsince air bubbles or amorphous carbon can be trapped during the process.

BRIEF SUMMARY OF THE DISCLOSURE

In an aspect, the present disclosure provides VDW films. VDW filmscomprise a plurality of monolayer transition metal dichalcogenide (TMD)films. The monolayer TMD films are stacked. By “stacked” it is meantthat each layer is in contact with at least one other layer and at leastpartially overlaps with one or both other (e.g., adjacent) layers. Theindividual TMD films interact via van der Waals forces. FIG. 1 presentsa representative VDW film with programmable compositions and cleaninterface.

VDW films can be free-standing films or disposed on a substrate orsurface.

Various substrates can be used. A substrate can be a solid substrate ora fluid (e.g., liquid) substrate.

A monolayer TMD film can include one or more transition metal sulfidesand/or one or more transition metal selenides. In various examples, amonolayer TMD film comprises MoS₂, WS₂, NbS₂, MoSe₂, WSe₂, MoTe₂, WTe₂,NbSe₂, or a combination thereof.

The scalability allows industrial application. The monolayer control(three-atom-thick) of a VDW film allows ultimate structural/compositioncontrol. The clean interfaces of a VDW films allow flat surface, highstructural stability, good optical properties and high mechanicalstrength (e.g., it allows suspension as atomically-thin membranes withhigh aspect ratio (lateral dimension/thickness) of 0.2 million or lessor 0.1 million or less, or 0.05 million or less).

In an aspect, the present disclosure provides apparatuses comprising oneor more VDW films of the present disclosure. The apparatuses have one ormore VDW films of the present disclosure and/or one or more VDW filmsmade by a method/methods of the present disclosure.

In an example, the apparatus is a hybrid structure and the apparatuscomprises, consists, or consists essentially of: optionally, asubstrate, and a plurality of VDW films, and a plurality of non-TMDlayers. One or more of the VDW films may be disposed on at least aportion of the substrate. The hybrid structures comprising non-TMDlayers can form atomically-thin circuits.

In an aspect, the present disclosure provides methods of making VDWfilms and/or apparatuses comprising VDW films. The methods are alsoreferred to herein as PVS processes/methods or VSDP processes/methods.In the methods, TMD monolayers are assembled by VDW interaction toprovide TMD VDW films.

The methods are based on mechanical release (e.g., dry peeling) of TMDmonolayers from a formation substrate and vacuum stacking of individualTMD films. The films are formed using van der Waals forces. The methodscan be used to make VDW films of the present disclosure. FIG. 2(a)illustrates examples of an instant VSDP process for generating highquality, programmable VDW films as shown in FIG. 1. A method of making aVDW film can further comprise formation of non-TMD monolayers.

In an aspect, the present disclosure provides devices. The devicescomprises one or more VDW films and/or one or more apparatuses of thepresent disclosure. Examples of devices include, but are not limited to,quantum electronic, mechanic, and photonic devices. Additional examplesof devices include, but are not limited to, tunnel devices, capacitors,diodes, membranes, optical windows, transparent electronic devices,optical devices, micro-electromechanical system devices, mechanicaldevices, optomechanical devices, optoelectrical devices, flexibleelectronics, and bio-compatible electronics.

DESCRIPTION OF THE FIGURES

For a fuller understanding of the nature and objects of the disclosure,reference should be made to the following detailed description taken inconjunction with the accompanying figures.

FIG. 1 presents a VDW film with programmable compositions and ultracleaninterface in accordance with the present disclosure.

FIG. 2 illustrates a VSDP process for generating high quality,programmable VDW films as shown in FIG. 1.

FIG. 3 illustrates the large scale layer-by-layer programmability inVSDP process with tunnel device/capacitor array over a large area.

FIG. 4 illustrates the composition programmability when N is fixed.

FIG. 5 illustrates generating the freestanding VDW membrane anddemonstrating its potential applications.

FIG. 6 shows the peeling process for 2-inch wafer scale MoS₂ monolayerfilm.

FIG. 7 illustrates spatial mapping of EELS corresponding to HAADFimages.

FIG. 8 illustrates optical absorption of MoS₂ films. a, spectracollected for N=10 with local measurement (diameter ˜50 μm) at differentlocations and global measurement for the whole sample (˜1 cm). Allspectra are similar, showing the optical uniformity of the film.

Inset: schematics of the sample and the measurement. b, absorption at532 nm as a function of N collected globally. Our result follows thetrend 1-TN with T=0.91. This roughly shows the N-control optically, incomplement to the electrical data in FIG. 3. Inset: schematics andphotos of the measured sample.

FIG. 9 illustrates Raman spectra of MoS₂ films for N=1 and 2. The twopeaks are the in-plane E12g and out-of-plane Alg mode. The peakseparation is related to the mechanical coupling in the materials. The˜21.5 cm-1 peak separation in N=2 indicates we have twist angles betweenlayers (typical number: monolayer ˜19; twisted bilayer ˜22; aligned (0or 60)˜23 cm-1 (8, 9).)

FIG. 10 shows characterizations of MoS₂ (9-layer) as a new dielectricmaterial. a, C-V curve and leakage (tunnel) current. It is shown thatthe capacitance maintains a constant within the range of [−1V, 1V],corresponding to a vertical electric field of E˜0.17 V/nm. Outside thisrange, the leakage current will significantly affect the measurement.Notice that this E strongly varies with N because the tunnel currentwould decrease exponentially. b, I-V curves before and after thebreakdown voltage around ˜2V (or,E˜0.34 V/nm). After the breakdownvoltage, it shows linear I-V curve at all voltage. This is anirreversible process.

FIG. 11 illustrates I-V curves of 6L-MoS₂ and 6L-WS₂ at large voltage.For MoS₂, the I-V curve is symmetric at all range. For WS₂, it isgenerally symmetric, with little bias-dependence(II(forward)/I(backward)|<2, in comparison to ˜10 for Mo/W.) For WS₂,the asymmetry might be an artifact because we use the fabricationprocess optimized for MoS₂ to process WS₂. For Mo/W, therefore, we useMoS₂ as the top 3 layers, which should help to preserve the intrinsicbehavior after device fabrication.

FIG. 12 shows schematics of a vacuum stacking tool.

FIG. 13 shows schematics of transferring process for suspended form.

DETAILED DESCRIPTION OF THE DISCLOSURE

Although claimed subject matter will be described in terms of certainembodiments, other embodiments, including embodiments that do notprovide all of the benefits and features set forth herein, are alsowithin the scope of this disclosure. Various structural, logical,process step, and electronic changes may be made without departing fromthe scope of the disclosure.

The present disclosure provides van der Waals (VDW) films and methods ofmaking VDW films. The present disclosure also provides apparatuses anddevices comprising VDW films.

The embodiments disclosed herein disclose a vacuum stack and dry peel(VSDP) method to achieve high quality VDW films in large scale.

High quality and large scale VDW films, assembled by monolayer buildingblocks, allows artificial design of the material at atomic level foradvanced devices such as quantum electronic, mechanic, and photonicdevices. The embodiments herein disclose newly-developed stackingtechnique for VDW films, which provides i) large scale processing up to2 inches (e.g., length, width, or diameter) based on the variousmonolayer TMD building blocks (e.g., MoS₂, MoSe₂, WS₂, and WSe₂) grownby MOCVD; ii) programmability in the VDW film via layer-by-layerassembly; and iii) ultraclean interface through dry peeling and stackingin vacuum (e.g., less than 200 mTorr). The high quality of theas-produced VDW film is investigated by using cross-sectional STEM andEELS with atomic resolution. The accurate programmability of our processis demonstrated electrically with tunnel device array in large scale,where the number of layer can be controlled, for example, with astandard deviation corresponding to less than 0.15 layers and tunnelresistance tuned up to ˜10⁴ with different compositions. With the use ofVSDP process, a new material platform is also demonstrated with freestanding VDW films, which are expected to provide, for example,atomically thin membrane mechanics, optics and electronics.

The instant methods can provide one or more of the following advantages:

i) use of ultrathin building block in large scale,ii) VDW films with composition programmability, without the need forchemical bonding and crystal matching between layers,iii) well-defined, intrinsic interfaces,iv) diverse material combination (including, for example, metal,semiconductor, superconductor and magnetic materials), andv) VDW films with layer thickness controllability at sub-nm scalethickness.

These advantages are expected to allow precise engineering of advanceddevices such as, for example, quantum electronic, mechanic, and photonicdevices.

In an aspect, the present disclosure provides VDW films. VDW filmscomprise a plurality of monolayer transition metal dichalcogenide (TMD)films. The monolayer TMD films are stacked. By “stacked” it is meantthat each layer is in contact with at least one other layer and at leastpartially overlaps with one or both other (e.g., adjacent) layers. Theindividual TMD films interact via van der Waals forces.

FIG. 1 presents a representative VDW film with programmable compositionsand clean interface. FIG. 1(a) shows a cross sectional annular darkfield TEM image of a VDW film, where the contrast of MoS₂ layers (dark)and WS₂ layers (bright) are clearly distinguishable due to the atomicnumber difference (also see in the contrast profile in FIG. 1(b)).Observations from FIG. 1(a) include the following. First, the VDW filmforms a superlattice, monolayer MoS₂/monolayer WS₂ alternative structureas we designed (schematics in FIG. 1(a)), and it shows monolayer scalecontrollability of composition and thickness. Second, the individuallayers show uniform, continuous, and straight monolayer and are parallelto each other. Third, the interlayers maintain extremely clean andbubble/wrinkle free interface with constant distance ˜0.638 nm, which isexpected value for MoS₂/WS₂ stacking with random crystal orientation(further discussion in Example 1). The corresponding elemental analysisalong the stacking direction from EELS are shown in FIG. 1(b). Themolybdenum concentration exactly matches the location of MoS₂ layer inFIG. 1(a), while sulfur is uniform everywhere for both MoS₂ and WS₂.Carbon, on the other hand, is not detectable under our instrumentresolution inside the stack, which also supports the cleanliness of theinterface. The above analysis confirms that our stacking process providethe composition controllability at monolayer scale in programmablemanner, and an interface quality without unwanted defects. The interfacemay be configured to have no detectable carbon residues, atomicallysharp interface, and/or no bubbles/wrinkles. For example, the interfacemay include less than one carbon chunk (particle) or air bubble per a 2μm×2 μm area. The cleanliness level may be a root mean square (RMS)roughness smaller than 300 pm and/or less than 0.1% carbon residuebetween layers.

A VDW film can comprise different kinds of TMD monolayer to formheterostructures (e.g., superlattices). A VDW film can be aheterostructure. For example, a heterostructure comprises two or moreTMD monolayers having different composition. Non-limiting examples ofheterostructures include ABCDBCDA, ABC, AB, AC, and the like (e.g.,where A, B, and C are TMD layers/monolayers having differentcomposition). A VDW film can be a superlattice. For example, asuperlattice comprises alternating heterostructures. A non-limitingexample of a superlattices is ABABABABAB (e.g., where A and B are TMDlayers/monolayers having different composition).

VDW films can be free-standing films or disposed on a substrate orsurface. Various substrates can be used. A substrate can be a solidsubstrate or a fluid (e.g., liquid) substrate. In various examples, thesubstrate comprises or consists of at least one of Al₂O₃, SiO₂, silicon(Si), or other metal or metalloid oxide(s). In various examples, thesubstrate comprises or consists of a polymeric material or polymer(e.g., polyethylene terephthalate). In an example, the substrate is skin(e.g., mammal skin such as, for example, human skin.

In another example, the substrate is an aqueous substrate (e.g., water).For example, where the substrate is an aqueous substrate (e.g., water) aVDW film is formed on a substrate, e.g., SiO₂, and then the VDWfilm/SiO₂ is contacted with an aqueous medium (e.g., water). The VDWfilm will float on the surface of the aqueous medium (e.g., water), butthe SiO₂ substrate will sink in the aqueous medium.

A monolayer TMD film can include one or more transition metal sulfidesand/or one or more transition metal selenides. In various examples, amonolayer TMD film comprises MoS₂, WS₂, NbS₂, MoSe₂, WSe₂, MoTe₂, WTe₂,NbSe₂, or a combination thereof.

A VDW film can have desirable cleanliness. For example, a VDW films haveone particle of carbon per 2 micron×2 micron area. In another example, aVDW film has less than 0.1% by weight carbon (e.g., carbon particlesand/or hydrocarbons) between layers (e.g., at the interface betweenlayers). In yet another example, a VDW film has no detectible carbon(e.g., carbon particles and/or hydrocarbons). Carbon can be detected bymethods known in the art. In various examples, carbon is detected byelectron energy loss spectroscopy (EELS), high-angle annular dark field(HAADF), or scanning transmission electron microscopy (STEM) imaging.

A VDW film has desirable surface roughness. In an example, a VDW filmhas a root mean square (RMS) roughness less than 300 μm. In anotherexample, a VDW film has a root mean square (RMS) roughness less than 200μm. The surface roughness can depend on the surface roughness of thegrowth substrate. In an example, a VDW film has a root mean square (RMS)roughness less than 300 pm, so long as the growth substrate does nothave a RMS roughness of 300 pm or greater. In another example, a VDWfilm has a root mean square (RMS) roughness less than 200 pm, so long asthe growth substrate does not have a RMS roughness of 200 pm or greater.In various examples, a desired surface roughness, which can be greaterthan 300 pm, is intentionally produced.

A VDW film can have a desirable amount of defects. For example, a VDWfilm has less than one bubble (e.g., air bubble) and/or wrinkles per 2micron×2 micron area. In another example, a VDW film has not observablebubbles and/or wrinkles at the monolayer TMD film interface(s). Bubbleand/or wrinkles can be detected by methods known in the art. In variousexamples, bubbles and/or wrinkles are detected by atomic forcemicroscopy (AFM), optical microscopy, scanning electron microscopy(SEM), Raman spectroscopy, or use of a tunnel device or capacitordevice.

A VDW film can have desirable material quality of each layer and/oroverall film (e.g. electrical properties, optical properties). Forexample, each layer in a VDW film can be optimized before stacking tohave high mobility of, for example, 30 cm²V⁻¹s⁻¹ or greater, and highphotoluminescence intensity. For example, a VDW film can sustain highbreakdown of voltage up to ˜0.5 V/nm or higher.

The scalability allows industrial application. The monolayer control(three-atom-thick) of a VDW film allows ultimate structural/compositioncontrol. The clean interfaces, which can be ultra-clean interfaces, of aVDW films allow flat surface, which can be an ultra-flat surface, highstructural stability, good optical properties (e.g. less optical loss)and high mechanical strength (e.g., it allows suspension asatomically-thin membranes with high aspect ratio (lateraldimension/thickness) of 0.2 million or less or 0.1 million or less, or0.05 million or less).

In an aspect, the present disclosure provides apparatuses comprising oneor more VDW films of the present disclosure. The apparatuses have one ormore VDW films of the present disclosure and/or one or more VDW filmsmade by methods of the present disclosure.

In various examples, an apparatus comprises, consists, or consistsessentially of: optionally, a substrate, and one or more VDW films. Oneor more of the VDW films may be disposed on at least a portion of thesubstrate.

In an example, the apparatus is a hybrid structure and the apparatuscomprises, consists, or consists essentially of: optionally, asubstrate, and a plurality of VDW films, and a plurality of non-TMDlayers. One or more of the VDW films may be disposed on at least aportion of the substrate. The hybrid structures comprising non-TMDlayers can form atomically-thin circuits.

Non-TMD layers can be formed by methods known in the art. For example,non-TMD layers can be formed by spin-coating (e.g., organic materials),thermal/e-beam evaporation (e.g., metal and oxide materials), sputtering(e.g., metal materials), ALD (e.g., oxide materials), Langmuir-Blodgetttechnique (e.g., nanocrystals and quantum dots), dip-coating (e.g.,organic materials, nanocrystal, metal-porphyrin molecules, and metalorganic framework compounds), physical vapor deposition (e.g.,metal-porphyrin molecules). These steps can be carried out before and/orin between individual TMD monolayer formation (e.g., individualpeel-and-stack steps for each newly-added layers) to form, for example,TMD monolayer/non-TMD layer/TMD monolayer/non-TMD layer/ . . . hybridstructures.

Non-TMD layers (e.g., films) are disposed on a VDW film. Non-TMD layersare stacked vertically, layer by layer, along with the TMD monolayerbuilding blocks. Non-limiting examples of non-TMD layers includes layerssuch as metal layers (e.g., metals such as, for example, Au, Ag, Al, Nb,Ni, and the like), oxide layers (e.g., non-metal and metalloid oxidefilms such as, for example, hafnium oxides, silicon oxides, aluminumoxides, and the like), organic (e.g., organic polymer films) films, andself-assembled nanostructures (e.g., metal-porphyrin molecules, metalorganic framework compounds, covalent organic frameworks compounds).Each non-TMD layer is separated from other non-TMD layers by at leastone VDW film.

Various substrates can be used. Non-limiting examples of varioussubstrates are provided herein.

Various VDW films can be used. In the case where the apparatus hasmultiple VDW films, the films can have the same or different nominalcomposition and/or the VDW films can be free-standing films and/or VDWdisposed on at least a portion of a substrate.

In an aspect, the present disclosure provides methods of making VDWfilms and apparatuses. The methods are also referred to herein as PVSprocesses/methods and VSDP processes/methods. In the methods, TMDmonolayers are assembled by VDW interaction to provide TMD VDW films.

The methods are based on mechanical release (e.g., dry peeling) of TMDmonolayers from a formation substrate and vacuum stacking of individualTMD films. The films are formed using van der Waals forces. The methodscan be used to make VDW films of the present disclosure. In an example,a method does not use a solvent (e.g., an organic solvent).

In an example, a method of making a VDW film comprises: providing aplurality of large area transition metal dichalcogenide (TMD) monolayers(e.g., a plurality of large area transition metal dichalcogenide (TMD)monolayers) on a substrate (a formation substrate); dry peeling at leastone of the TMD monolayers from the substrate; layer-by-layer stacking atleast one of the TMD monolayers (e.g., by dry peeling at least one ofthe TMD monolayer from the formation substrate and transferring the TMDmonolayer to a substrate under vacuum to form a Van der Waals (VDW)film.

FIG. 2(a) illustrates examples of an instant VSDP process for generatinghigh quality, programmable VDW films as shown in FIG. 1. In step 1, aseries of wafer scale TMD monolayers on SiO₂/Si substrate are preparedby MOCVD. Other techniques besides MOCVD that enable mechanical peelingalso can be used. Step 2, the initial layer (L0) is separated from thegrowth substrate by dry peeling with poly(methyl methacrylate) (PMMA)tape or other thermal release tape (TRT). Step 3, L0/TRT and the nextTMD layer (L1) on substrate are put into a vacuum box and pressed intocontact in vacuum. Step 4, L1/L0/TRT is peeled off from the substrate.The last two steps can be repeated until the desired number of layers(N) is reached. In the final step 5, the N-layer TMD film is releasedfrom TRT to any target, in either supported or suspended form, forfurther characterizations and applications.

The structure formed using the VSDP process is different from usingother techniques. First, the controllability of thickness or compositionis at a monolayer level (sub-nm), which cannot be achieved by MOCVD orALD. Second, the various material combination of heterostacking isallowed for our process since each interfaces combined by weak van derWaals interaction. However, MOCVD or ALD method are only allowed forspecific material combination under consideration of their epitaxialrelation, lattice constant, chemical bonding, or surface energy.

In VSDP process, the high quality MOCVD-grown films can be completelyseparated from the growth substrates with solely mechanical force (e.g.,“dry peeling”) due to extremely low interaction with the growth surface.The dry peeling can ensure an ultra-clean bottom surface without anychemicals such as etchant or solvent for the following stacking. Inaddition, stacking individual monolayer in the vacuum (vacuum stacking)further improves the interface quality by avoiding air exposure when thestamp layers contact as-grown target samples. The vacuum stacking anddry peeling are repeatable for multi-stacked films as long as TMD-TMDinteraction is stronger than TMD-growth substrates, which is governed bystacking condition (see Example 1).

FIG. 2(b-d) demonstrate the resulting VDW film from VSDP process isscalable, ultra-clean, programmable and universal to various TMDs. FIG.2(b) displays photos of three layer MoS₂ stacked in wafer scale on TRTduring the process. The inset photo shows the initial layer L0 from a 2inch wafer on the TRT after the first peeling. Two more MoS₂ layers L1,L2 from 1 inch square substrates are then stacked as in the main photo,where clear contrast between different N's can be observed. Furtheroptical characterizations of our large scale VDW films depending on Nare presented in SI.

FIG. 2(c) shows the surface roughness of the VDW films by atomic forcemicroscopy (AFM) to manifest the effect of vacuum stacking. Images aretaken at the bottom surface of 3-layer MoS₂ films when the stacking isconducted in vacuum (left) or in ambient (right). As shown, the rightimage has bubble-like features (RMS˜700 pm) on the surface, while theleft image appears to be smooth (RMS˜270 pm). Therefore, we concludethat vacuum-stacking greatly improves the cleanness at the interface inour film, which may be necessary to build these VDW films withconsistent quality.

The cross-sectional STEM image in FIG. 2(d) shows MoSe₂/MoS₂/WS₂ VDWfilms prepared by VSDP. Even though there is a lattice mismatch betweenMoSe₂ and MoS₂ (˜4%), as well as an interlayer rotation between MoS₂ andWS₂, VDW films still forms without misfit dislocations (Example 1). Ourprogrammable VSDP can be applied to be universal TMD monolayers in anycombination regardless their lattice difference and rotation angle.

The membrane can include alternating MoS₂ and WS₂ monolayers or othercombinations of different monolayers. For example, the TMD stack caninclude half MoS₂ and half WS₂.

The large area TMD stack can be at least three, at least six, or atleast nine layers thick. In an example, twenty layers are stacked. Evenlarger numbers of stacked layers are possible.

We demonstrated the large scale layer-by-layer programmability in VSDPprocess with tunnel device/capacitor array over a large area shown inFIG. 3(a), since high material quality is required for the devices. Asan example, the tunnel current is exponentially sensitive to i) barrierwidth, determined by N and ii) barrier height, determined by thecomposition of the VDW film. Therefore, controllable tunnelcharacteristics over large area requires process with precise anduniform control in thickness, doping as well as compositions in largescale, which has not been achieved by any means to date. Here, VSDPenables us to fabricate device array in large scale by standardphotolithography (see Example 1), and the accurate programmability isconfirmed in FIGS. 3 and 4.

We first show our layer-by-layer control of N with verticalgold/MoS₂/gold sandwiched structure for tunnel device. Tunnel devicesform when the VDW film is sandwiched vertically by metals with workfunction deep inside its band gap (e.g., schematics in FIG. 3(b) forgold and MoS₂). In FIG. 3(b), all representative devices indeed showgeneral tunneling I-V characteristic with exponentially decreasingcurrent when N increases. Quantitatively, we plot the zero-biastunneling resistance (R₀A) as a function of N in FIG. 3(c). Ourexperiment data (diamonds) follow the tunneling equation (dash line)when the barrier height is ϕ_(B)=0.5 eV (detail in Example 1). Moreover,the device array with N=7 (inset) shows excellent spatial uniformitywith the uncertainty less than 35% of the average, equivalent to assmall as 0.15 layer variation. Note that all devices have working areaas big as 5×5 μm² and distributed over ˜5×5 mm² on chips for each N,showing the large area uniform control and consistent reproducibility ofVSDP process.

In FIG. 3(d), we further confirm our control over N through capacitancemeasurement. Similarly, the N-dependence of MoS₂ capacitors (emptydiamonds) shows small variations and follows the parallel platecapacitor equation (dash line) when dielectric constant is ε_(MoS2)=2.9,close to previous reported value for monolayer MoS₂ (detail in Example1). The consistent values and trend in both cases above shows that VSDPprocess indeed provides uniform control over N at monolayer level.Moreover, devices shown here come from many batches of MOCVD growth andfabrication, which further indicates the consistency of VSDP process.

In parallel, FIG. 4 shows the composition programmability when N isfixed. Three different VDW films are programmed as examples: MoS₂ (6layer), WS₂ (6 layer), and MoS₂(3 layer)/WS₂(3 layer), noted as Mo/W.Their tunneling I-V curves at nearly zero bias are measured in FIG.4(a), which show exponentially different tunnel resistance according totheir band structure (see schematics). For MoS₂ and WS₂, despite theirsimilar band gap, the ˜0.4 eV higher band offset of WS₂ makes it ahigher barrier. Therefore, WS₂ shows 10,000 times larger tunnelresistance at the same thickness compared to MoS₂. On the other hand,the intermediate resistance can be generated by the Mo/W film as shown,because the effective barrier height of Mo/W at small bias istheoretically the average of its components, MoS₂ and WS₂. The agreementbetween experiment results and theoretical band structures here confirmsour control over the compositions.

Furthermore, based on the programmed band structure, the ultra-thin Mo/W(˜3 nm) is theoretically predicted to behave as themetal-insulator-insulator-metal (MIIM) tunnel diode at large bias.Indeed, this diode behavior is observed in FIG. 4(b). The operation ofMIIM is illustrated as the inset schematics: (ii) shows the zero-biasregime, as in FIG. 4(a); in (iii), a large positive voltage V is appliedto MoS₂ and bends part of the conduction band below the fermi level,allowing electrons to tunnel from gold into MoS₂; in contrast, electronsstill have to go through the full barrier at −V in (i), due to thehigher band offset in WS₂. The different effective barrier widththerefore attributes to the asymmetric I-V curve. This behavior of Mo/Wmixture not only shows our programmability, but points out a way viaVSDP for new physical properties that cannot be achieved with singleelement system.

FIG. 1-4 confirms the layer-by-layer programmability of compositions andultraclean interface between layers in our VDW films via VSDP process,which enable us to design artificial material with atomic control closeto fundamental limits. Moreover, VSDP process also allow us to separatethe programmable VDW films from the substrates as a free standingmembrane in large scale to take the advantage of its ultra-thinness.

In FIG. 5, we generate the freestanding VDW membrane and demonstrate itspotential applications. As shown in FIG. 5a , the VDW film istransferred on a TEM chip with 1×1 mm² hole at the center (seeschematics). Bottom photo shows an example of fully suspended 7-layerMoS₂ films over the hole with clear yellowish surface. Our suspendedmembrane is successfully demonstrated since the TMD monolayer can becompletely separated from the substrate and stacked without holes,wrinkles and cracks (Example 1). In addition, the film is only ˜5 nmthick over 1 mm hole, giving one of the highest aspect ratio(length:thickness) of 2×10⁶:1, which can provide an atomically thinmaterial platform in technologically relevant scale toward futuremembrane mechanics, optics and electronics as well as the integration ofthem.

In FIGS. 5(b) and (c), we demonstrate applications of the VDW membranethat can be potentially integrated with others for practical uses. InFIG. 5(b), patterned gold on fused silica is imaged clearly through theMoS₂ membrane (schematics on top) by optical microscope under whitelight illumination. The high transparency, inherent from itsultra-thinness, makes it a promising platform for optical windows ortransparent electronics. In FIG. 5(c), the scanning electron microscope(SEM) image shows the freestanding cantilever array patterned by focusedion beam (FIB). FIB allows us to pattern arbitrary shape on atomicallythin membrane, which would lead to new class of micro-electromechanicalsystems (MEMS).

The VSDP processes disclosed herein illustrate new methods for precisematerial programming down to atomic level with ultraclean interface. Itsprocess works up to wafer scale and allows final VDW films in eitherform of on-substrate or substrate-free. Our method is expected to beuniversal to any layered materials or even patterned atomically thincircuitry, as long as the materials can be separated from substrateswith a clean method. The new capability brought in by VSDP process may,in principle, accelerate the use of layered materials for physicalsystems and state-of-the-art technology, which is potentially beneficialto both academia and industry.

TMD layers/monolayers can be formed by methods known in the art. TMDlayers/monolayers can be formed using metal-organic chemical vapordeposition (MOCVD). For example, a TMD layer/monolayer or TMDlayers/monolayers are formed by methods disclosed in U.S. patentapplication Ser. No. 15/130,407 (titled “MONOLAYER FILMS OFSEMICONDUCTING METAL DICHALCOGENIDES, METHODS OF MAKING SAME, AND USESOF SAME”), which was published as U.S. Patent Application PublicationNo. US 2016/0308006, the disclosure of which with respect to formationof TMD layers/monolayers is incorporated herein by reference.

A formation substrate is any substrate on which a TMD monolayer can beformed and from which a TMD monolayer can be mechanically released.Non-limiting examples of formation substrates include silica substrates,silica (e.g., SiO₂), such as, for example, quartz, PECVD grownSiOx/silicon), other oxides or nitride substrates such as, for example,Al₂O₃ substrates (e.g., Al₂O₃ single crystal substrates), Al₂O₃/SiO₂/Sisubstrates, HfO₂/SiO₂/Si substrates, SiN/SiO₂/Si substrates, and thelike. In an example, a formation substrate comprises an external SiO₂surface).

Dry peeling can be carried out by forming a handle layer on at least aportion of a TMD monolayer disposed on formation substrate. Examples ofhandle layers include, but are not limited to, organic polymer layers,metal layers, metal oxide layers, and hetero-materials (e.g., thinpolymer layer disposed on thick flexible polymer layer and a brittlelayer (e.g., a thin metal layer) disposed on a thick flexible polymer.It is desirable that the handle layer conformally covers the portion ofthe TMD monolayer. It is desirable that the handle layer has aflexibility and interaction with the TMD monolayer that allows the TMDmonolayer to be removed from the formation substrate and transferred toa substrate. An organic polymer handle layer can have a thickness in themillimeter range. A metal layer or metal oxide layer can have athickness of 100 nm or less.

A handle layer can be formed using a tape. Non-limiting examples oftapes include thermal release tapes such as for example,poly(methylmetacrylate) (PMMA) and poly(vinyl acetate) (PVA) releasetapes. For example, dry peeling a TMD monolayer includes attaching tape(e.g., PMMA/thermal release tape) onto a first TMD monolayer and peelingat least one of the TMD monolayers from the substrate with the tape.

Transferring a TMD monolayer to a substrate, other TMD monolayer, or anon-TMD layer can be carried out by mechanically removing a TMDlayer/monolayer from a formation substrate using a handle layer, puttingthe TMD layer/monolayer on the substrate, other TMD monolayer, or anon-TMD layer and applying a mechanical force to the handle layer andremoving the handle layer to provide a TMD monolayer disposed on the TMDmonolayer on the substrate, other TMD monolayer, or a non-TMD layer.

Mechanical force can be applied to the handle layer in various ways. Inan example, mechanical force is applied to the handle layer using astamper.

The transferring is carried out under vacuum (e.g. in a vacuumenvironment). In an example, the transferring is carried out a pressureof 1 Torr or less. In another example, the transferring is carried out apressure of less than 200 mTorr. In yet another example, thetransferring is carried out a pressure of 100 mTorr or less. In yetanother example, the transferring is carried out a pressure of 1 Torr to0.1 mTorr. Without intending to be bound by any particular theory, it isconsidered that lower pressures provide desirable VDW films.

Heating can be used to facilitate removing the handle layer from the TMDmonolayer after transfer to the substrate, other TMD monolayer, or anon-TMD layer. For example, the handle layer is removed at temperaturesof 50° C. to 200° C., including all integer ° C. values and rangestherebetween.

The VDW film can be formed using a layer-by-layer method resulting in aVDW film comprising a plurality of stacked TMD monolayers. In variousexamples, a VDW film comprises 2, 3, 4, 5, 6, 7, 8, 9, or 10 stacked TMDmonolayers. In various examples, a VDW films comprises at least three,at least six, or at least nine TMD monolayers. The number of layers isnot limited by any part of the method and can be add up to any number asdesired. In various examples, a VDW film comprises 1-3,000 stacked TMDmonolayers, including all integer number of TMD monolayers and rangestherebetween. In various examples, a VDW film comprises 1-2,000,1-1,000, 1-500, 1-100, 1-50, 2-2,000, 2-1,000, 2-500, 2-100, or 2-50stacked TMD monolayers, In various examples, a VDW film comprises 2 ormore, 3 or more, 4 or more, 5 or more, or 10 or more stacked TMDmonolayers.

A method of making a VDW film can further comprise formation of non-TMDmonolayers. Non-TMD layers can be formed using methods known in the art.Non-limiting examples of methods of forming non-TMD layers include byspin-coating (organic materials), thermal/e-beam evaporation(metal/oxide), sputtering (metal), ALD (oxide), Langmuir-Blodgetttechnique (nanocrystals, quantum dots), dip-coating (organic materials,nanocrystal, metal-porphyrin molecules, metal organic frameworkcompounds), physical vapor deposition (metal-porphyrin molecules). Thesesteps can be carried out before or in between the peel-and-stack stepsfor each newly-added layer to form TMD/non-TMD/TMD/non-TMD/ . . . hybridstructure.

A method can comprise one or more additional steps. For example, amethod further comprises one or more heating steps. The heating, whichis different than heating to remove a handle layer, comprises heatingthe substrate and one or more TMD layers, and one or more non-TMDlayers, if present. In various examples, the substrate and one or moreTMD layers, and one or more non-TMD layers, if present, is heated at atemperature of 40-200° C. The heating conditions, including, forexample, temperature and vacuum level) does not release the handlelayer. Without intending to be bound by any particular theory, it isconsidered that such heating improves adhesion between TMD monolayersand TMD monolayer to non-TMD layer adhesion. For example, a methodfurther comprises superacid treatment on the VDW films after transfer toa substrate and/or another VDW film.

The steps of the method described in the various embodiments andexamples disclosed herein are sufficient to produce the VDW films,apparatuses, or devices of the present disclosure. Thus, in an example,a method consists essentially of a combination of the steps of themethods disclosed herein. In another example, a method consists of suchsteps.

In an aspect, the present disclosure provides devices. The devicescomprises one or more VDW films and/or one or more apparatuses of thepresent disclosure.

Examples of devices include, but are not limited to, quantum electronic,mechanic, and photonic devices. Additional examples of devices include,but are not limited to, tunnel devices, capacitors, diodes, membranes,optical windows, transparent electronic devices, optical devices,micro-electromechanical system devices, mechanical devices,optomechanical devices, optoelectrical devices, flexible electronics,and bio-compatible electronics.

Non-limiting examples of devices include:

i) VDF films with engineered thermal and thermoelectric properties inthe vertical direction. For measurement accuracy, the thermalmeasurements require high quality, large-scale samples with significantthickness (e.g., greater than 15 layers);ii) novel Josephson junction (JJ) arrays using TMD barriers to replacemetal oxides. The use of TMD barriers instead of metal oxides (e.g.,aluminum oxide) of JJ arrays is expected to provide independent controlof the tunnel resistance and capacitance while increasing the operationtemperature (e.g., using Nb, which has higher T_(c) than Al);iii) high current H-shape selector arrays, which can be used in, forexample, low-power memory devices. H-shape tunnel barrier layerscurrently generated by ALD technique are usually thick (greater than 5nm), limiting the magnitude of the tunnelling current. The instant PVSprocess can produce much thinner H-shape potential that it expected toproduce exponentially larger tunnelling current or smaller operationvoltage for lower power consumption;iv) hybrid TMD quantum optical resonators, which would enable, forexample, light-mediated remote coherent interactions among multiple TMDresonators;v) VDW membranes/windows (e.g., greater than mm scale) (e.g.,large-scale VDW membranes/windows) with desirable quality, controlledthickness, and uniformity. Such VDW membranes/windows can replaceconventional SiN windows for, for example, lower optical loss and colortunable transparency; andvi) Photovoltaic devices with designed broadband absorptionspectrum/external quantum efficiency by using a VDW film or films withselected composition. This will provide higher power conversionefficiency.

The following Statements provide examples of VDW films, apparatuses,methods, and devices of the present disclosure:

Statement 1. A method of forming a VDW film (e.g., a VDW film having ahomostructure, heterostructure, or superlattice structure) of thepresent disclosure (e.g., a VDW film comprising one or more TMDmonolayers), the method comprising: providing a formation substratehaving one or more transition metal dichalcogenide (TMD) monolayersdisposed on the formation substrate; dry peeling as described herein atleast one (e.g., one) of the TMD monolayers from the formationsubstrate; transferring the TMD monolayer(s) to a substrate under vacuumto form a Van der Waals (VDW) film; and, optionally, repeating theproviding, dry peeling, and transferring a desired number of times toform a VDW film comprising a plurality of TMD monolayers on thesubstrate.Statement 2. A method according to Statement 1, further comprisinggrowing (forming) the TMD monolayers as described herein (e.g., usingmetal-organic chemical vapor deposition (MOCVD)).Statement 3. A method according to any one of the preceding Statements,where the dry peeling at least one of the TMD monolayers comprisesattaching a handle layer as described herein to the TMD monolayer andremoving (e.g., peeling) at least one of the TMD monolayers from thesubstrate using the handle layer.Statement 4. A method according to Statement 3, wherein the handle layeris a tape (e.g., a thermal release tape).Statement 5. A method according to Statement 4, where the tape isPMMA/thermal release tape.Statement 6. A method according to any one of Statements 3-5, whereafter transferring the TMD monolayer to the substrate the TMD monolayeris released from the handle layer.Statement 7. A method according to any one of Statements 3-6, whereinthe releasing includes heating the handle layer to a releasetemperature.Statement 8. A method according to any one of the preceding Statements,wherein the transferring comprises attaching at the handle layer to astamper and contacting the TMD monolayers with the substrate or anotherTMD monolayer using the stamper.Statement 9. A Van der Waals (VDW) film (e.g., a VDW film having ahomostructure, heterostructure, or superlattice structure) of thepresent disclosure (e.g., VDW film comprising one or more transitionmetal dichalcogenide (TMD) monolayers), where the VDW film has nodetectible carbon and/or less than one bubble defect and/or wrinkledefect per 2 micron×2 micron area and/or has less than 0.1% by weightcarbon (e.g., carbon particles and/or hydrocarbons) between layers(e.g., at the interface between layers).Statement 10. A VDW film according to Statement 9, where the VDW filmcomprises one or more TMD monolayers of the present disclosure (e.g.,one or more TMD monolayers selected from MoS₂ monolayers, WS₂monolayers, MoSe₂ monolayers, WSe₂ monolayers, MoTe₂ monolayers, WTe₂monolayers, NbSe₂ monolayers, and combinations thereof).Statement 11. A VDW film according to any one of Statements 9-10, wherethe VDW film comprises a plurality of TMD monolayers (e.g., 2-3000 TMDmonolayers, 2-2000 TMD monolayers, 2-1000 TMD monolayers, 2-500 TMDmonolayers, 2-100 TMD monolayers, or 2-50), or three or more TMDmonolayers, six or more TMD monolayers, or nine or more TMD monolayers.Statement 12. A VDW film according to any one of Statements 9-11, wherethe VDW film comprises at least two TMD layers having differentcomposition.Statement 13. A VDW film according to any one of Statements 9-12, wherethe film is disposed on a substrate.Statement 14. An apparatus comprising one or VDW film of the presentdisclosure (e.g., one or more VDW film of any one of Statements 9-12) ormade according to a method of the present disclosure (e.g., madeaccording to any one of Statements 1-8).Statement 15. An apparatus according to Statement 14, where theapparatus further comprises one or more non-TMD layers, wherein anindividual non-TMD layer is not in contact with another non-TMD layer.Statement 16. A device comprising one or more VDW film and/or one ormore apparatus of the present disclosure (e.g., one or more VDW film ofany one of Statements 9-13 and/or one or more apparatus of any one ofStatements 14-15).Statement 17. A device according to Statement 16, where the device is atunnel device, a transparent electronic device, an optical device, amicro-electromechanical systems device, a mechanical device, aphotovoltaic device, an optomechanical device, an optoelectrical device.Statement 18. A device according to Statement 16, wherein the device isa capacitor, a diode, a membrane, or an optical window.Statement 19. A device according to Statement 16, wherein the device isa Josephson junction (JJ) array, a high current H-shape selector array,or quantum optical resonator.

The following examples are presented to illustrate the presentdisclosure. They are not intended to limiting in any matter.

Example 1

This example provides a description of apparatuses and methods of thepresent disclosure.

The realization of high quality and large scale VDW films, assembled bymonolayer building blocks, would allow artificial design of the materialat atomic level for advanced devices such as quantum electronic,mechanic, and photonic devices. We describe a newly-developed stackingtechnique for VDW films, which provides i) large scale processing up to2″ based on the various monolayer TMD building blocks (MoS₂, MoSe₂, WS₂,and WSe₂) grown by MOCVD; ii) programmability in the VDW film vialayer-by-layer assembly; iii) ultraclean interface through dry peelingand stacking in vacuum. The high quality of the as-produced VDW film isinvestigated by using cross-sectional STEM and EELS with atomicresolution. The accurate programmability of our process is demonstratedelectrically with tunnel device array in large scale, where the numberof layer can be controlled with standard deviation corresponding to lessthan 0.15 layers and tunnel resistance tuned up to ˜10⁴ with differentcompositions. With the use of VS process, a new material platform isalso demonstrated with free standing VDW films, which would provide thebasis toward future atomically thin membrane mechanics, optics andelectronics.

Material architecture with precise composition and thicknesscontrollability has been a central interest for modern science andtechnology. For instance, epitaxial layer-by-layer deposition includingmolecular beam epitaxy (MBE) and pulsed laser deposition (PLD) is one ofthe prominent techniques, and it has led to applications fromlight-emitting diode (LED), quantum cascade laser to new physical system(1-4). Recently, the discovery of atomically thin layered materials suchas graphene, h-BN and transition metal dichalcogenides (TMD) triggered anovel concept, sequential layer-by-layer assembly of individualultra-thin building blocks, to achieve ultimate controllability down toatomic scale with diverse material combination. The materials generatedby this process are named as van der Waals (VDW) films since thebuilding blocks are assembled by non-epitaxial VDW interaction betweeneach layers. Currently, these atomically thin building blocks can bereadily grown as uniform monolayer in large scale through chemical vapordeposition (CVD). However, existing methods to assemble them into VDWfilms are primarily developed for small scale samples from exfoliationmethod, which is limited to micrometer size without scalability.Furthermore, the cleanness at the stacking interface is not guaranteedwith current methods since air bubbles or amorphous carbon can betrapped during the process. Herein we describe our new process based onTMD building blocks that incorporates i) large-scale monolayer TMDsgrown by the recently developed metal-organic chemical vapor deposition(MOCVD) and ii) vacuum stack (VS) method, which we specifically designto realize large scale assembly with ultraclean interface. With the useof this process, we successfully demonstrate the fabrication of VDWfilms in large scale while having material control down to atomic levelwith ultraclean interfaces.

FIG. 1 presents our representative VDW film with programmablecompositions and ultraclean interface. The TEM samples are prepared byion milling and focused ion beam (FIB) from randomly selected regions ona large scale VDW film. FIG. 1(a) shows a cross sectional annular darkfield TEM image of our VDW film, where the contrast of MoS₂ layers(dark) and WS₂ layers (bright) are clearly distinguishable due to theatomic number difference (also see in the contrast profile in FIG.1(b)). There are three main observations from FIG. 1(a). First, the VDWfilm forms a superlattice, monolayer MoS₂/monolayer WS₂ alternativestructure as we designed (schematics in FIG. 1(a)), and it showsmonolayer scale controllability of composition and thickness. Second,the individual layers all appear as uniform, continuous, and straightmonolayer, and are parallel to each other. Third, the interlayersmaintain extremely clean and bubble/wrinkle free with constantinterlayer distance ˜0.638 nm, which is expected value for MoS₂/WS₂stacking with random crystal orientation. The corresponding elementalanalysis along the stacking direction from EELS are shown in FIG. 1(b)and FIG. 7. The molybdenum concentration (green) exactly matches thelocation of MoS₂ layer in FIG. 1(a), while sulfur (yellow) is uniformeverywhere for both MoS₂ and WS₂. Carbon (red), on the other hand, isnot detectable under our instrument resolution inside the stack, whichalso supports the cleanness of the interface. The above analysisconfirms that our stacking process provide the compositioncontrollability at monolayer scale in programmable manner, and excellentinterface quality without unwanted defects.

FIG. 2(a) illustrates our VS process for generating high quality,programmable VDW films as shown in FIG. 1. There are five steps for theprocess as below. I, a series of wafer scale TMD monolayers on SiO₂/Sisubstrate are prepared by MOCVD. II, the initial layer (L0) is separatedfrom the growth substrate by dry peeling (FIG. 6) with pmma/thermalrelease tape (TRT). III, L0/TRT and the next TMD layer (L1) on substrateare put into a vacuum box and pressed into contact in vacuum. IV,L1/L0/TRT is peeled off from the substrate. The last two steps, III andIV, can be repeated until the desired number of layers (N) is reached.V, the N-layer TMD film is released from TRT to any target, in eithersupported or suspended form, for further characterizations andapplications.

The process introduced above is specially designed to reach scalability,programmability and ultraclean interface of VDW films. First, the MOCVDis suitable for wafer scale growth of monolayer TMD building blockssince the gaseous MO precursor can be precisely controlled uniformlyover entire substrates (10). In addition, it can be applied to generalTMD materials by combination of metal and chalcogenide precursors.Second, the extremely low interaction of monolayer TMDs with the growthsurface allows the repeatable stack-and-peel step as long as adjacentTMD-TMD interaction is stronger than TMD-growth substrates, which isgoverned by stacking condition. This enables us to achieve programmableVDW films with arbitrary composition and desired N. Third, themechanical peeling guarantees ultra-clean bottom surface without anychemicals such as etchant or solvent. This together with the followingstacking of individual monolayer in the vacuum significantly improvesthe interface quality by avoiding air exposure that could introduceamorphous carbon and air bubble when the stamp layers contact as-growntarget samples and generate the interfaces.

FIG. 2(b-d) are the demonstrations of the above capability in theresulting VDW films. FIG. 2(b) displays photos of three layer MoS₂stacked in wafer scale on TRT during the process. The inset photo showsthe initial layer L0 from a 2″ wafer on the TRT after the first peeling.Two more MoS₂ layers L1, L2 from 1″ square substrates are then stackedas in the main photo, where clear contrast between different N's can beobserved. Further optical characterizations of our large scale VDW filmsdepending on N are presented in FIGS. 8 and 9.

FIG. 2(c) shows the surface roughness of the VDW films by atomic forcemicroscopy (AFM) to manifest the effect of vacuum stacking. Images aretaken at the bottom surface of 3-layer MoS₂ films when the stacking isconducted in vacuum (left) or in ambient (right). As shown, the rightimage has bubble-like features (RMS˜700 pm) on the surface that are alsoreported in previous work, while the left image appears to be smooth(RMS˜270 pm). Therefore, we conclude that vacuum-stacking improves thecleanness at the interface in our film, which helps build these VDWfilms with consistent quality.

The cross-sectional STEM image in FIG. 2(d) shows MoSe₂/MoS₂/WS₂ VDWfilms prepared by VS. Even though there is a lattice mismatch betweenMoSe₂ and MoS₂ (˜4%), as well as an interlayer rotation between MoS₂ andWS₂, VDW films still forms without misfit dislocations. It presents ourcomposition programmability can be applied to be universal TMDmonolayers in any combination regardless their lattice difference androtation angle.

Based on the process, we are now able to control, at monolayer level,the electrical properties of the VDW film in the out-of-plane directionwith large scale in-plane uniformity. Specifically, we demonstrate suchcapability in two ways: controllability of N (FIG. 3), andprogrammability of composition (FIG. 4). For this purpose, we fabricatevertical metal-VDW film-metal devices (schematics, FIG. 3(a)) to measurethe electrical transport in the out-of-plane direction, where the VDWfilm being sandwiched is controllably varied in the experiment. On thecontrary, to exam the in-plane uniformity of our process, we fabricatethe devices as an array by standard photolithography shown in FIG. 3(a).Here, the devices are designed with 5×5 μm² sandwiched area anddistributed over ˜5×5 mm² on fused silica for each N as the testplatform for large-scale uniformity (detail in Method).

First, we demonstrate the electrical properties control depending on N.Tunnel devices are used as the test platform because the tunnel currentis exponentially sensitive to the barrier shape such as width(determined by N) and height (determined by the composition). In FIG.3(b), we carefully design the gold/N layer (NL−) MoS₂/gold sandwichedstructure to form the tunnel band structure (see schematics) and measuretheir I-V curves with N=3, 5, 7. It is qualitatively observed that,first, all representative devices show the non-linear I-V characteristicof tunneling. Second, the current decreases exponentially by ˜10² witheach additional two layers. These two features together indicate that wehave the tunneling across the MoS₂ barrier as described by theschematics, and, indeed, the N can be varied. Notice that the design ofthe band structure may be critical here: for example, the tunnelcharacteristic may disappear if Ti is used as the contact metal. Theimportance of such band alignment design will be further discussed inFIG. 4 as well as in Example 1.

Quantitatively, statistical analysis is shown in FIG. 3(c). Here, weplot the statistics of the zero-bias tunneling resistance (R₀A) of thedevices to each N (detail in SI). Our experiment data show consistentincrement by a factor of 10 for each additional layer from N=3 to 7.Moreover, this trend follows the theoretical tunneling model (dash line)(17) when the single fitting parameter, the barrier height ϕ_(B), is 0.5eV. The agreement here shows that the MoS₂ indeed forms a good tunnelbarrier, because otherwise the tunnel resistance can deviateexponentially if N is not controlled in the sandwiched area of thedevice. For large scale uniformity, as shown in the inset, the devicearray with N=7 over the ˜5×5 mm² area shows very small uncertainty,which is less than 35% of the average, equivalent to a variation of only0.15 layers. The results above indicate that we achieve the highcontrollability of N and the large-scale uniformity in our process.

In FIG. 3(d), the control over N is further confirm up to N=11 withcapacitance measurement, which provides another thickness-sensitivecharacterization. Similarly, the N-dependence of MoS₂ capacitors (emptydiamonds) shows small variations and consistent trend that follows theparallel plate capacitor equation (dash line) when the single fittingparameter, the dielectric constant E MoS₂, is 2.9. This value is alsoclose to previous reported value for monolayer MoS₂ (detail in Example 1and FIG. 10), showing our control over MoS₂ as a good dielectric withdesigned N's up to arbitrary number.

In parallel, we can also control the electrical properties via theprogrammability of composition, which allows us to design the bandalignment. In FIG. 4, we show such capability using the same geometry inFIG. 3(a) but having three different VDW films with the same N: MoS₂,WS₂, and MoS₂/WS₂ (3-layer/3-layer, noted as Mo/W hereafter). FIG. 4(a)first shows their tunneling I-V curves at nearly zero-bias. When theMoS₂ is replaced by the WS₂, we are able to tune the zero-biasresistance up to ˜10⁴ times, from 1 MΩ·μm² to 10 GΩ·μm². This hugetuning can be theoretically explained by the higher band offset in WS₂,which therefore forms a higher barrier (see schematics). Meanwhile, Mo/Wshows 100 MΩ·μm² that is well between the other two, indicating that weare also able to fine-tune the tunnel resistance with their mixtureaccording to the band structure and tunnel model (see schematics andExample 1).

Besides tuning existing properties, Mo/W also forms the distinctasymmetric tunnel barrier that can lead to new properties. As shown inFIG. 4(b), at large bias the tunneling I-V curve shows large asymmetry,with forward bias (+V) current being 10 times higher than the reverse(−V) at V=1.4 Volt (WS₂ grounded as schematics shown). In comparison,MoS₂ and WS₂ both show symmetric curves in the same voltage range (seeFIG. 11). We attribute this diode behavior to themetal-insulator-insulator-metal (MIIM) tunnel diode, since tunneling isthe dominant transport in this ultra-thin film (˜4 nm) device. Theoperation of MIIM is illustrated as the inset schematics: at forwardbias (iii), the large voltage Von MoS₂ bends part of the conduction bandbelow the fermi level, allowing electrons to tunnel from gold into MoS₂.In contrast, electrons still have to go through the full barrier at −Vat reverse bias (i), due to the higher band offset in WS₂. The differenteffective barrier width at opposite bias therefore attributes to theasymmetric I-V curve. In FIG. 4, both demonstrations show properties aswe designed based on theoretical band structure, which cannot beachieved if there is control failures such as unwanted doping orinterlayer mixture of the elements in the VDW film. Accordingly, thecontrol of the material properties here indicates that our processindeed provides excellent programmability of the compositions.

FIG. 1-4 confirms the layer-by-layer programmability of compositions andultraclean interface between layers in our VDW films via VS process,which enable us to design artificial material with atomic control closeto fundamental limits. Moreover, VS process also allow us to separatethe programmable VDW films from the substrates as a freestandingmembrane in large scale to take the advantage of its ultra-thinness.

In FIG. 5, we generate the freestanding VDW membrane and demonstrate itspotential applications. As shown in FIG. 5a , the VDW film istransferred on a TEM chip with 1×1 mm² hole at the center (seeschematics). Bottom photo shows an example of fully suspended 7-layerMoS₂ films over the hole with clear yellowish surface. Our suspendedmembrane is successfully demonstrated since the TMD monolayer can becompletely separated from the substrate and stacked without holes,wrinkles and cracks. In addition, the film is only ˜5 nm thick over 1 mmhole, giving one of the highest aspect ratio (length:thickness) of2×10^(6:1), which can provide a novel atomically thin material platformin technologically relevant scale toward future membrane mechanics,optics and electronics as well as the integration of them.

In FIGS. 5(b) and (c), we demonstrate applications of the VDW membranethat can be potentially integrated with others for practical uses. InFIG. 5(b), patterned gold on fused silica is imaged clearly through theMoS₂ membrane (schematics on top) by optical microscope under whitelight illumination. The high transparency, inherent from itsultra-thinness, makes it a promising platform for optical windows ortransparent electronics. In FIG. 5(c), the scanning electron microscope(SEM) image shows the freestanding cantilever array patterned by FIB.FIB allows us to pattern arbitrary shape on atomically thin membrane,which would lead to new class of micro-electromechanical systems (MEMS).

In conclusion, the VS process presented here illustrates a new methodfor precise material programming down to atomic level with ultracleaninterface. Its simple process works up to wafer scale and allows finalVDW films in either form of on-substrate or substrate-free. Our methodis expected to be universal to any layered materials or even patternedatomically thin circuitry, as long as the materials can be separatedfrom substrates with a clean method. The new capability brought in by VSprocess may, in principle, accelerate the use of layered materials fornovel physical systems and state-of-the-art technology, which ispotentially beneficial to both academia and industry.

Growth of TMD films. Wafer scale monolayer films of MoS₂, WS₂, MoSe₂,WSe₂ were grown by metal organic chemical vapor deposition (MOCVD) (1).Molybdenum hexacarbonyl (MHC), tungsten hexacarbonyl (THC), diethylsulphide (DES), and dimethyl selenide (DMSe) are selected as chemicalprecursors for Mo, W, S, and Se respectively, and introduced to thefurnace in gas phase. H₂ and Ar are injected to the chamber usingseparate lines. The optimum growth parameters for ML TMD films are asfollows. We use a total pressure of ˜10 Torr, growth temperature of 550°C. and growth time of 26 hrs. The flow rate of precursors are 0.01 sccmfor MHC or THC, 0.4 sccm for DES, or DMSe, 5 sccm for H₂, and 150 sccmfor Ar, which were regulated by individual mass flow controllers (MFCs).NaCl is loaded in the upstream region of the furnace, whichsignificantly increases the grain size.

Stacking. (1) Fabrication of Initial Layer L0

L0 is used as the stamp layer and the process is as follow: Spin coatingof PMMA (Poly-methyl methacrylate, 495K, 4% diluted in anisole) for 90second at 4000 rpm on as-grown monolayer TMD films (MLTMD) sitting onSiO₂/Si. Baking 10 min at 180° C. using hot plate, followed by attachingthermal release tape (TRT) manufactured by Nitto on PMMA/MLTMD/SiO₂/Si.TRT/PMMA/MLTMD(L0) is separated from the substrate via mechanicallypeeling, which granted it the ultraclean bottom surface. PMMA can bereplaced by any thin film that can be conformally deposited on the TMDsurface, such ALD SiO₂, HfO₂, CVD Si, and thermal evaporated Au. Thisprocess can be generally applied to MOCVD grown monolayer TMD film, suchas MoS₂, WS₂, MoSe₂, and WSe₂.

(2) Stacking in the Vacuum Box

As shown in FIG. 12, we use a specially designed vacuum stacking toolincluding vacuum sealed box, vacuum pump, linear motion vacuumfeedthrough, and heating unit underneath of the vacuum box. Stackingprocess is as follow: Mount TRT/PMMA/L0 at top holder, and put anotheras grown monolayer films on the bottom stage of vacuum box. Lower thetop holder to make contact between L0 and as-grown monolayer L1 onSiO₂/Si on the bottom stage using z-motion linear vacuum feedthrough.Stay for 10 mins Lift the top holder with stacked sample. After thestacking process, L0/SiO₂/Si is readily attached to TRT/PMMA/L0. Thestacking process operates when the chamber is evacuating to less than200 mTorr, and heating at 150° C.

(3) Re-Peeling and Re-Stacking

To improve separation yield, following steps are carried out beforestarting next round of ‘peel’ and ‘stack’ process: 1) release the usedTRT from PMMA/L0/L1/SiO₂/Si by heating at 110° C., ambient condition. 2)do additional annealing at 180° C. for 10 min after removing TRT. 3)attach new TRT on PMMA/L0/L1/SiO₂/Si. After replacing the TRT, thebottom of the sample (i.e., Si) is attached on a glass slide usingdouble side tape. The stacked film (TRT/PMMA/L0/L1) is separated fromthe substrate using mechanical peeling again. The repeatable process of‘stack’ and ‘peel’ allows us to generate L2, L3, . . . , LN.

(4) Transfer and Releasing

(i) Supportive Form

LN is transferred on any target substrates using vacuum stackingprocess. Then TRT is removed by heating at 110° C. at ambient condition.PMMA on multi-stack VDW films can be removed by either way of highvacuum (<10⁻⁶ Torr) annealing at 325° C., or soaking to acetone after anadditional annealing at 180° C. in ambient for 30 mins and cooling down.

(ii) Suspended Form

As shown in FIG. 13, suspended PMMA/VDW films are generated by using TRTwith a punched hole inside. After separation of PMMA/VDW films from thesubstrate by mechanical peeling, the suspended PMMA/VDW films can betransferred onto the target frame with gradual heating from roomtemperature to 180° C. to allow the PMMA on VDW films to melt andconformally cover the frame. The remaining PMMA outside of the frame iscut by knife. The PMMA is removed by high vacuum (<10⁻⁶ Torr) annealingat 325° C.

TEM analysis. STEM specimen preparation and imaging: A cross section ofthe specimen was prepared by using a standard lift-out procedure in adual-beam FEI Strata 400 focus ion beam system with a final milling at 2keV. Afterwards, the specimen was baked in an ultrahigh vacuum chamberat 130° C. for 8 hours to clean the specimen. After baking, the specimenwas transferred to a Nion Ultra-STEM 100 operated at 60 keV. The imagingcondition was similar to that in (2). For HAADF-STEM images, the beamconvergence angle was ˜35 mrad, with a probe current of ˜70 pA. Theacquisition time was 8 μs per frame and we sum 10 frames. The EELSspectrum and maps were acquired with an energy dispersion of 0.25eV/channel using a Gata Quefina dual-EELS Spectrometer. A linearcombination of power laws (LCPL) was used to fit and subtract thebackground. The EELS false-color composition maps were created byintegrating the S-L2,3 edge, C-K edge, Mo-M4,5 edge and Si-L2,3 edge.All EELS analysis was done with open-source Cornell Spectrum Imagersoftware (3).

Optical measurements. VDW films of different N are transferred to fusedsilica substrate for the optical absorption measurement.Photoluminescence and Raman spectroscopy are done with SiO₂/Sisubstrate.

Optical absorption: Measurements are done in transmission mode withDUV-Vis-NIR hyper-spectral microscope described in (6) and ShimadzuUV-Vis-NIR Spectrometer for local and global measurement, respectively.Spot size for the hyperspectral microscope is ˜50 μm in diameter whileit is ˜1 cm for Shimadzu Spectrometer. We measure the transmitted lightintensity at the two regions, VDW films on substrate (Iv) and baresubstrate (Is), and calculate the fractional change in the transmittance(δ_(T)) as (Iv−Is)/Is. δT is approximately linked to the absorption (A)by δ_(T)=(2/n_(s)+1)×A, where n_(s) is the refractive index of fusedsilica here.

Photoluminescence: The photoluminescence (PL) measurements are performedwith a 532 nm excitation laser under ambient conditions. The PL spectrafrom the sample are collected by an imaging spectrometer with a CCDcamera, and the PL images were taken directly using bandpass filterswith the center wavelength corresponding to 1.9 eV for MoS₂.

Raman spectroscopy: Measurements are performed with green laser (532 nm)in InVia Confocal Raman microscope (Renishaw) at room temperature.Spatial resolution ˜1 μm.

Device fabrication. Devices geometry is as shown in FIG. 3(a). For thefabrication, we start with e-beam evaporation of 5 nm Ti/40 nm Au onfused silica chips (1.5×1.5 mm²). The bottom electrodes are defined bystandard photolithography (PL) method, followed by gold solutionetching. In order to have good VDW film transfer, the bottom electrodehas to be very clean. Therefore, after dissolving the photoresist (PR)in Microposit Remover 1165, we treat the substrate with O₂ plasma at 400W for 3 minutes in Glen 1000 to make sure there is minimum residue. TheVDW film (typically ˜5×5 mm²) with programmed N and composition is thentransferred to the bottom electrode as described in FIG. 2(a) and above,and the pmma supporter is removed in acetone. It is observed that evenafter pmma is removed, the initial layer is more n-doped than as-grown.Therefore, we adopt the previously reported superacid treatment (S2) onthe VDW film to eliminate the doping from pmma at this point.Afterwards, top electrodes are fabricated by e-beam evaporation of 40 nmAu, patterned by PL and gold solution etching, and remove of PR. Bottomgold electrodes covered by VDW films are intact after the solutionetching owing to the good peeling yield and its inert chemicalreactivity. In the next step, we also etch away the part outside of thecrossing area by PL for patterning and SF₆/O₂ plasma in Oxford Plasmalab80+ for etching. In the final step, super-acid treatment is performedagain to make sure the effect maintains after all the process.

Electrical measurements. All the electrical measurement are done inambient condition at room temperature with Karl Suss PSM6 Probe Stationusing W probe tip (SE-20 TB, Signatone). For I-V characterization, theprobe station is coupled to high precision source measurement units(SMUs) (Keithley, 236 SOURCE MEASUREMENT UNITS), voltage source(Keithley, 213 QUAD VOLTAGE SOURCE) and trigger (Keithley, 2361 TRIGGERCONTROLLER). The I-V measurement on tunnel devices (FIG. 3(b)(c) andFIG. 4) is performed in four probe geometry for N≤6 and two probe forN≥6 to minimize error from contact resistance and instrument impedance,respectively. No significant difference between the two geometry isobserved for N=6. For capacitance measurement, the probe station iscoupled to Keithley C-V system (Keithley, 590 CV ANALYZER/230PROGRAMMABLE VOLTAGE SOURCE/5951 REMOTE INPUT COUPLER). All measurementsare done in two probe geometry, and the parasitic capacitance of theinstrument is measured and subtracted from the capacitance results.

Tunnel equation for zero-bias resistance. The zero-bias resistance (R₀A,in Ω·μm¹) from experiment is extracted by linear fitting to each I-Vcurve at very small bias (between ±0.01 V). Theoretically, R₀A isdescribed as the following equation.

${R_{0} \cdot A} = {\frac{h^{2}{Nt}_{{MoS}_{2}}}{e^{2}\sqrt{2m\;\phi_{B}}}{\exp\left( \frac{4\pi\sqrt{2m\;\phi_{B}}{Nt}_{{MoS}_{2}}}{h} \right)}}$

for V<<ϕB, where h is the planck constant, e and m electron charge andeffective mass, t the thickness and ϕ_(B) is the average barrier heightof the barrier (thus also applicable to FIG. 4(a)). This equation showsthat R₀A is independent of the applied bias V, and is only dependent onbarrier parameters such as the barrier width and height. Therefore, itserves as a good number for comparison.

In FIG. 3(c), the dash line comes from the above equation with setparameters of MoS₂ (t=0.65 nm, m=0.35 m0 (m0 the electron's free mass))and one fitting parameter rϕ_(B). When ϕ_(B)=0.5 eV, the equation showsthe best fit. Note that we do the fitting by first converting into logscale for calculation and then convert it back to linear scale forplotting to avoid over-weighting of the higher resistance data points.Other values such as the average and uncertainty are all calculated inthe same way for the same reason.

MoS₂ dielectric constant and its application as new dielectric material.In FIG. 3(d), all devices (N=7 to N=11) can be fitted by a singledielectric constant ε_(MoS2)=2.9. Although this number is close toreported value for monolayer MoS₂, ε_(MoS2) increases as a function ofN. However, this N-dependence is not observed in our devices. Onesignificant difference in our MoS₂ film is the random twist anglesbetween the layers, as pointed out in FIG. 2(d) and the Raman spectrumbelow, while previous dielectric studies are all conducted on TMD withaligned crystal axes (i.e., 0° or 60°). The distinct crystal structurescan likely result in the different dependence of dielectric constanthere. However, support is needed for the hypothesis.

Our capacitance experiment here also indicated that TMDs can serve asgood dielectric materials. There are several advantages of TMDs incomparison to common dielectrics such as oxide and hexagonal boronnitride (hBN). For oxide dielectrics, they generally degrade theperformance of 2D materials. For example, the mobility of graphene isreported to be degraded on silicon oxide due to the ubiquitous danglingbonds on the surface as charge scattering centers. In contrast, 10 timesbetter mobility is observed with hBN as the substrate since its surfacehas dramatically less dangling bonds. However, for hBN it cannot beproduced with thickness homogeneity up to wafer scale so far, limitingthe application of hBN to small scale devices. On the contrary, TMDsalso has a dangling bond-free surface similar to hBN. Moreover, based onour method, we can produce TMD dielectric with controlled thickness andwafer-scale uniformity, making it more promising for practicalapplications. The basic characterizations (C-V curve, leakage currentand breakdown voltage) are presented below in FIG. 10.

Although the present disclosure has been described with respect to oneor more particular embodiments, it will be understood that otherembodiments of the present disclosure may be made without departing fromthe scope of the present disclosure. Hence, the present disclosure isdeemed limited only by the appended claims and the reasonableinterpretation thereof.

1.-19. (canceled)
 20. A method of forming a film comprising one or moreTMD monolayers, the method comprising: providing a formation substratehaving one or more transition metal dichalcogenide (TMD) monolayersdisposed on the formation substrate; dry peeling at least one of the TMDmonolayer from the formation substrate; and transferring the TMDmonolayer to a substrate under vacuum to form a film.
 21. The method ofclaim 20, further comprising repeating the providing, dry peeling, andtransferring a desired number of times to form a film comprising aplurality of TMD monolayers on the substrate.
 22. The method of one ofclaim 20, wherein the dry peeling at least one of the TMD monolayer(s)comprises attaching a handle layer to a TMD monolayer disposed on theformation substrate and peeling at least one of the TMD monolayer(s)from the formation substrate using the handle layer.
 23. The method ofclaim 22, wherein the handle layer comprises a tape.
 24. The method ofclaim 23, wherein the tape comprises PMMA/thermal release tape.
 25. Themethod of claim 22, wherein after transferring the TMD monolayer(s) tothe substrate the TMD monolayer(s) is/are released from the handlelayer.
 26. The method of claim 22, wherein the releasing includesheating the handle layer to a release temperature.
 27. The method ofclaim 22, wherein the transferring comprises attaching the handle layerto a stamper and contacting the TMD monolayer(s) with the substrate oranother TMD monolayer using the stamper.
 28. The method of claim 20,wherein the transferring is carried out under a pressure of 1 Torr orless.
 29. A film comprising one or more transition metal dichalcogenide(TMD) monolayer(s), and having a heterostructure comprised of at leasttwo atomically thin monolayer building blocks, wherein the film has lessthan one bubble defect and/or wrinkle defect per 2 micron×2 micron area.30. The film of claim 29, wherein an interlayer between two adjacent TMDmonolayers is a clean, bubble-free and wrinkle-free interface comprisinga substantially constant distance.
 31. The film of claim 29, wherein thefilm comprises two or more TMD monolayers, each of the two or more TMDmonolayers is selected from MoS₂ monolayer, WS₂ monolayer, MoSe₂monolayer, WSe₂ monolayer, MoTe₂ monolayer, WTe₂ monolayer, or NbSe₂monolayer.
 32. The film of claim 29, wherein the film comprises aheterostructure comprising at least two atomically thin monolayer TMDbuilding blocks made from at least two different TMD materials.
 33. Thefilm of claim 29, wherein the at least two atomically thin monolayerbuilding blocks are configured to stack and interact via van der Waals(VDW) forces to form the film.
 34. An apparatus comprising one or morefilm(s) of claim
 29. 35. The apparatus of claim 34, wherein theapparatus further comprises one or more non-TMD layer(s), wherein anindividual non-TMD layer is not in contact with another non-TMD layer.36. A device comprising one or more film(s) of claim
 29. 37. The deviceof claim 36, wherein the device is a tunnel device, a transparentelectronic device, an optical device, a micro-electromechanical systemsdevice, a mechanical device, a photovoltaic device, an optomechanicaldevice, or an optoelectrical device, and/or wherein the device is acapacitor, a diode, a membrane, an optical window, a Josephson junction(JJ) array, a high current H-shape selector array, or a quantum opticalresonator.
 38. An atomically-thin circuit comprising at least a portionof a film of claim
 29. 39. A heterostructure comprising: a firsttransition metal dichalcogenide (TMD) monolayer; a second TMD monolayerdisposed on the first TMD monolayer; wherein each of the first and thesecond TMD monolayer are atomically thin, wherein the heterostructurecomprises less than one bubble defect and/or wrinkle defect per 2micron×2 micron area, and wherein each of the first and the second TMDmonolayer comprise different materials.